A. Field of the Invention
The present invention relates to a wide angle zoom lens system with two lens groups. This wide angle zoom lens system has a compact construction and obtains a wide view angle and a long back focus distance by using a front lens having negative refractive power in the first lens group.
B. Description of the Related Art
A normal focal length lens has a focal length approximately equal to the diagonal length of the recorded image. Thus, in still picture photography, a 50 millimeter (mm) lens is generally considered a normal lens for 35 mm film which has a negative diagonal of approximately 42 mm. A wide angle lens has a focal length less than a normal lens, while a long or telephoto lens has a focal length greater than a normal lens.
In a zoom lens system, the overall focal length is adjustable with the widest angle of view at the shortest focal length and the narrowest angle of view at the longest focal length.
Recently, compact "lens shutter" cameras have been introduced. These cameras require a compact, wide angle zoom lens with high magnification. If a panoramic function is desired, the lens shutter cameras require a back focus distance longer than that of an ordinary compact zoom lenses to facilitate attachment of the panoramic mechanism.
In a zoom lens system composed of two lens groups, having a big zooming ratio and a wide view angle, the back focus distance is shortened at a wide angle position and the distance between the two lens groups is shortened at a telephoto position. These are critical problems.
In order to solve these problems, Japanese Patent laid-open No. 93-19166 discloses a wide angle zoom lens having a first lens group in which the first lens has a negative refractive power.
If a lens having a negative refractive power is used in the first lens group to obtain a long back focus distance in these conventional zoom lenses having a wide view angle and high magnification, the refractive power of the lens having a positive refractive power increases such that the sum of spherical aberration and Petzval's sum also increases. Thus, as the magnification increases, it is difficult to compensate for aberrations.